The purpose of this project is to study the molecular mechanisms of chemoreceptor regulation in rat carotid body (CB). Exposure to hypoxic conditions (5% 02) causes an immediate drop in pa02 to about 35 mm Hg, a decrease in dopamine (DA) content of CB, but fails to change the norepinephrine (NE) concentration in this tissue. These changes are followed by an activation of c-nucleotide phosphodiesterase (PDE) and an increase in RNA polymerase II-dependent RNA-synthesis. After 24 hours tyrosine hydroxylase (TH) activity in CB is increased. The increase in PDE activity, RNA-synthesis and TH activity requires intact innervation of the CB. BIBLIOGRAPHIC REFERENCES: Hanbauer, I., Lovenberg, W. and Costa, E.: Induction of tyrosine-3-monooxygenase in carotid body of rats exposed to hypoxic conditions. Neuropharmacology 16: 277-282, 1977. Hanbauer, I.: Regulation of tyrosine hydroxylase in carotid body. In Costa, E. and Gessa, G.L. (Eds.): Advances in Biochemical Psychopharmacology. New York, Raven Press, 1977, Vol. 16, pp. 275-280.